This invention relates to a design of a heating unit of a thermal ink jet and more specifically to a layout of a power driver of the heating unit for providing more current drive capability to a heating element in the heating unit.
Generally, an ink jet printing system has a printhead which comprises one or more ink filled channels communicating with an ink supply chamber at one end and having a nozzle opening at the opposite end. A heating element is placed at the bottom of a bubble containment chamber which in turn is located at a predetermined distance from the nozzle. A flow of an electric current through the heating element, which is a resistive element, heats up the heating element vaporizing the ink in the chamber and forming a bubble. As the bubble grows, the ink is ejected out of the nozzle. By stopping the current flow, the heating element cools off causing the bubble to collapse. While the bubble is collapsing, the ink at the vicinity of the nozzle is pulled in resulting in drop ejection by separation of the ink outside of the nozzle from the ink inside of the nozzle.
The width of a heating unit is limited to the nozzle width and the width of the nozzle is limited tothe pixel width. As the number of the pixels in each printing system grows, the allocated space to each pixel decreases thereby decreasing the allocated width of each nozzle and each heating unit. A 300 dot per inch (DPI) printing allocates 84 micron layout width and a 600 DPI printing allocates 42 micron layout width.
Referring to FIG. 1, a heating unit 10 comprises a resistor 12 for heating and a current driver, usually a MOSFET (metal oxide silicon field-effect transistor) 14. The drain 16 of the n-channel MOSFET 14 is connected to a power supply V.sub.DD through the resistor 12, the source 18 of the MOSFET 14 is connected to ground potential, and the gate 20 is connected to an input drive circuitry (not shown) through an input V.sub.i.
With a growing need for faster printing engines, the need for more powerful heating units is also growing. To fabricate a more powerful heating unit 10, a current driver with a higher current drive capability is required. Moreover, to fabricate a current driver with a higher current drive capability a larger MOSFET 14 is needed which in turn requires a larger gate 20. To layout a larger gate 20, the layout area of the heating element has to be increased. Since the width of the layout area of the heating unit is limited, only the length of the layout area can be increased.
Referring to FIG. 2, the conventional MOSFET current drivers are laid out to have a gate 20 with a vertical serpentine pattern. The serpentine pattern is laid out to have a plurality of long strips 21 parallel to each other along the length of the layout of the current driver 14. Also, the serpentine pattern has a plurality of short strips 15 which connect the long strips to each other in such a manner to generate a long continuous gate. The serpentine pattern of the conventional layout is called vertical since the long strips 21 are laid out along the length of the layout of the current driver 14. The drain area 16 and source area 18 are located between the long strips 21 of the gate 20 in an interlace form. The drain metal 17 has a plurality of fingers 22 and the source metal 19 has a plurality of fingers 24. The drain fingers 22 and the source fingers 24 are laid out in such a manner that they are in an interlace relationship with each other and each drain finger 22 is located above a drain area 16 and each source finger 24 is located above a source area 18. Each drain finger 22 has plurality of contacts 26 with the drain area 16 underneath the drain finger 22. Each source finger 24 has plurality of contacts 28 with the source area 18 underneath the source finger 24.
The conventional MOSFET current driver layout is prone to have shorts or defects. The multiplicity of the drain and the source fingers and the multiplicity of the spaces between the drain fingers 22 and the source fingers 24 gives rise to the probability of having a short or a defect. Therefore, the fabrication yield of this design is very low.